Defending champion Serena Williams top seed for U.S. Open

NEW YORK (Reuters) - World number one Serena Williams has been top-seeded for the U.S. Open where she will be seeking to win the women's singles title for a third consecutive year, the United States Tennis Association announced on Wednesday.

Williams has emerged triumphant five times in the year's last grand slam tournament, clinching the crown in 1999, 2002, 2008, 2012 and 2013.

Though the American has not progressed beyond the fourth round at a grand slam in 2014 mainly due to back trouble and a virus, she served notice she is back to her best by winning her fifth WTA title of the season in Cincinnati on Sunday.

With her U.S. Open victory last year, Williams earned her 17th grand slam singles title, putting her sixth in the all-time list with only Steffi Graf (22), Chris Evert (18) and Martina Navratilova (18) ahead of her in the professional era.

Romania's world number two Simona Halep, a surprise finalist at the French Open earlier this year, has been seeded second for the 2014 U.S. Open at Flushing Meadows with big-serving Czech Petra Kvitova, the Wimbledon champion, seeded third and Poland's Agnieszka Radwanska fourth.

Reigning French Open champion Maria Sharapova of Russia, who won the 2006 U.S. Open, is seeded fifth.

World number three Li Na of China, a semi-finalist at Flushing Meadows last year, withdrew from the U.S. Open last month because of a knee injury.

The U.S. Open draw will be conducted at the USTA Billie Jean King National Tennis Center on Thursday before the tournament starts on Monday.